Estrogens are involved in the modulation of the cardiovascular system, yet their effects in young women remains largely unknown. Women who undergo ovulation induction treatments attain extremely high estrogen concentrations during a very short time period. The aim of the present study was to evaluate the effects of an acute increase in estrogens on the autonomic nervous system modulation of heart rate variability (HRV). A total of 27 women undergoing ovulation induction and 14 normally menstruating women were prospectively studied. HRV was assessed during nadir and peak estrogen using time domain and power spectral density analyses. A significant increase in high-frequency spectral power (243 +/- 77 vs. 188 +/- 73 ms(2)/Hz, P < 0.01) with a significant decrease in the ratio of low to high-frequency power was observed during estrogen peak in women undergoing induction of ovulation. The acute increase in estrogen in women undergoing ovulation induction was associated with vagal activation and altered sympathovagal balance.